#include <bits/stdc++.h>
using namespace std;

ifstream fin ("cifre.in");
ofstream fout("cifre.out");

#define cin  fin
#define cout fout

int a, b, c, k;

int solutie = 0;

bool verifica(int x)
{
    int contor = 0;
    while(x)
    {
        if(x % 10 == c) contor++;
        x /= 10;
    }

    if(contor >= k)
    {
        return true;
    }
    return false;
}

int main()
{
    int i;
    cin >> a >> b >> c >> k;

    for(int i=a; i<=b; i++)
    {
        if(verifica(i)) solutie++;
    }

    cout << fixed << setprecision(4) << double((double)solutie / (double)(b -a + 1));
}